A number of systems have been used for the extraction and/or removal of compounds and analytes from solid or semi-solid matrices for quantification and identification.
Soxhlet extraction has been in use for over 100 years. In this technique, the extraction of analytes takes place at or close to room temperature, over a period of several hours to several days, and generally uses a large volume of solvent to sample ratio. Fast Soxhlet extractions are also done at the boiling point of the solvent; this system is sold under the tradename "SOXTEC" and is manufactured by Perstorp, Inc. A similar system is marketed under the tradename "SOXTHERM" and is made by ABC Laboratories. For example, an automated Soxhlet extraction technique is used in the Environmental Protection Agency (EPA) method 3541 for the extraction of organic analytes from soil, sediment, sludges and waste solids.
Microwave extraction has also been used, which provides shorter extraction times due to faster heat up times. U.S. Pat. No. 4,554,132 describes an apparatus for the use of microwave for drying the sample combined with solvent extraction at atmospheric pressure in unsealed vessels. Other techniques have been described for the preparation of samples for chromatography, ICP (Inductively Coupled Plasma Emission Spectroscopy) and amino acid analysis (U.S. Pat. No. 4,554,132; P. Hocquellet and M.-P. Candillier, Analyst, 116:505-509 (1991); K. Ganzler, A. Salgo and K. Valko, J. Chromatography, 371:299-306 (1986); K. Ganzler, J. Bati and K. Valko, Akademiai Kiado, Chromatography '84, Budapest, Hungary, H. Kalasz and L. S. Ettre, eds., pp.435-442 (1984); K. Ganzler, I. Szinai and A. Salgo, J. Chromatogr., 520:257-262 (1990); K. I. Mahan, T. A. Foderaro, T. L. Garza, R. M. Martinez, G. A. Maroney, M. R. Trivisonno and E. M. Willging, Anal. Chem., 59:938-945 (1987)) using microwave extraction in unsealed vessels.
Sealed vessels have also been described (refs 7-13) in conjunction with microwave extractions (L. A. Fernando, W. D. Heavner and C. C. Gabrielli, Anal. Chem., 58:511-512 (1986); L. B. Fischer, Anal. Chem., 58:261-263 (1986); H. M. Kingston and L. B. Jassie, Anal. Chem., 58:2534-2541 (1986); R. Rezaaiyan and S. Nikdel, J. of Food Science, 55:1359-1360 (1990); J. Nieuwenhuize, C. H. Poley-Vos, A. H. van den Akker and W. van Delft, Analyst, 116:347-351 (1991); M. B. Campbell and G. A. Kanert, Analyst, 117:121-124 (1992)). These sealed vessels allow the use of higher pressures and temperatures; for example, reported pressures vary from 40 psi (L. A. Fernando, W. D. Heavner and C. C. Gabrielli, Anal. Chem., 58:511-512 (1986); L. B. Fischer, Anal. Chem., 58:261-263 (1986)) to 3000 psi (W. Lautenschlaeger, Spectroscopy International, 2:18-22 (1990)). These systems are utilized to dissolve or digest the sample matrix completely, and typically in large volumes of solvent.
For example, microwave extraction has been used to extract additives and stabilizers from polyolefins (W. Freitag and O. John, Die Angewandte Makromoiekulare Chemie, 175:181-85 (1990); R. C. Nielson, J. Liq. Chromatogr., 14:503-519 (1991)). In these examples, the polyolefins are ground and added to an excess of solvent, heated in a microwave, and the solvent containing the analyte is analyzed. In some cases the solvent was evaporated prior to analysis.
U.S. Pat. No. 5,147,551 describes an apparatus used in extraction. A sample is placed in a sealed vessel with a frit. Solvent, which may be heated or unheated, is introduced into the vessel, which may also be heated. After a soak period, an inert gas is swept up through the frit and through the sample to remove the volatile analytes, and then the gas is analyzed, for example on a gas chromatograph.
The extraction of various analytes from a solid matrix samples using a fluid under elevated temperatures and pressures sufficient to cause the fluid to be in a supercritical condition is also well known and has been in use for many years (P. Capriel, A. Haisch and S. U. Kahn, J. Agric. Food Chem., 34:70-73 (1986); M. Schnitzer, C. A. Hindle and M. Meglic, Soil Sci. Soc. Am. J., 50:913-919 (1986); M. Schnitzer and C. M. Preston, Soil Sci. Soc. Am. J., 51:639-646 (1987)). Carbon dioxide, for example, is a commonly employed material for supercritical analyte extraction. The carbon dioxide will be held in a container or cell which is raised to a temperature and pressure which causes the carbon dioxide to operate as a supercritical fluid. While in the supercritical conditions, the fluid is forced through a porous sample to cause extraction of analytes from the sample. A wide range of samples and analytes are amenable to such supercritical extraction techniques.
It also has been found that the addition of a solvent to a supercritical fluid, in relatively low percentages, for example, 10% or less, will enhance the supercritical extraction process. While supercritical fluid extraction, with solvent augmentation, enhances the supercritical fluid extraction result, the temperatures and pressures at which the fluid is maintained in supercritical condition are greater than would be optimum for a pure solvent extraction.
Accordingly, it has been recently discovered that a highly effective solvent extraction process for the extraction of organic analytes from a solid matrix sample can be accomplished by maintaining an organic analyte in contact with a non-aqueous organic solvent system in an extraction cell under temperatures and pressures below supercritical conditions. This process is described in detail in commonly owned U.S. patent application Ser. No. 08/259,667, filed Jun. 14, 1994, and entitled "Accelerated Solvent Extraction System," which application is incorporated herein by reference in its entirety.
In most of the above-mentioned solvent extraction techniques, a container or vessel is required which enables a solvent to flow into the vessel, while maintaining a seal, to contact the analyte. Since the conditions of extraction are often conducted under elevated temperatures and pressures, the seal must adequately perform under a wide range of temperatures and pressures. One such integral high pressure and high temperature seal is disclosed in U.S. Pat. No. 5,193,703 which describes a pressure vessel having a tapered internal sealing surface adapted to receive a tapered mating seal. An outer body portion is formed to engage the vessel, while a sealing body, contained generally within the outer body portion, sealably engages the sealing surface at a distal end thereof. By applying an external force to a proximal end of the sealing body, which then slides relative the outer body portion, the distal end increasingly engages the sealing surface to form a seal.
One problem associated with this design is that the internal seal reduces the overall volume or capacity which the vessel would otherwise be capable of holding. During filling of the vessel, it is usually desirable to top-off the vessel with sample. Because the seal is internal, engagement of the seal against the sealing surface will displace the sample which causes spillage, and more importantly, reduces the internal volume.
Another disadvantage with this design is that the tapered sealing surface and mating tapered seal are not always true to one another. Hence, balancing problems may occur upon which the seal may become wedged against the tapered sealing surface. This ultimately results in separation problems between the tapered sealing surface and the seal.